OCA DA DH MEA MSA

What is OCH(A)?

OCH is the obstacle clearance height above the aerodrome level (and so has relevance when using QFE). OCA is the obstacle clearance altitude above mean sea level (and so has relevance when using QNH).

What is the minimum-height/altitude rule?

In order to comply with the instrument flight rules (IFR) both inside and outside controlled airspace, and without prejudice to the usual low-flying rules, the minimum height rule dictates that an aircraft should not fly at a height of less than a 1000 ft above the highest obstacle within a distance of 5 nautical miles of the aircraft unless

1. It is necessary for the aircraft to do so in order to land

2. The aircraft is flying on a route notified (NOTAM, AIP) for the purpose of this rule (this may include controlled airspace such as terminal control areas and airways)

3. The aircraft has been otherwise authorized by the competent authority

4. The aircraft is flying at an altitude not exceeding 3000 ft above mean sea level (MSL) and remains clear of clouds and in sight of the surface An additional International Civil Aviation Organization (ICAO) height rule increases this height to 2000 ft above the highest point in mountainous terrain.

What lighting designations are on air navigation (i.e., man-made) obstacles?

Obstacles greater than 492 ft (150 m) are lit by high-intensity flashing white lights by day and night. Any failed lights are NOTAMed. Obstacles less than 492 ft (150 m) but higher than 300 ft, are lit when the obstacle is considered significant with medium-intensity flashing red lights at night. These lights are not NOTAMed when they fail.
What is MSA? Minimum sector altitudes (MSAs) are published on

1. Instrument approach charts. They provide at least 300 m (1000 ft) vertical clearance within 25 nautical miles of the homing facility for the particular instrument approach. If the aircraft remains at or above the relevant MSA, then it should remain clear of terrain and obstacles as it tracks toward the aerodrome prior to commencing the approach. Some aerodromes have MSAs that apply to all sectors; however, most aerodromes have different MSAs for different sectors depending on the direction from which the aircraft is arriving and the terrain over which it must cross.

2. En route charts. These show grid moras (or minimum sector safe altitudes). When determining your current MSA or grid mora altitude from an en route chart, you should take the most restrictive MSA of all the adjacent grid moras to your present position.

What is MSA?

Minimum sector altitudes (MSAs) are published on:

1. Instrument approach charts. They provide at least 300 m (1000 ft) vertical clearance within 25 nautical miles of the homing facility for the particular instrument approach. If the aircraft remains at or above the relevant MSA, then it should remain clear of terrain and obstacles as it tracks toward the aerodrome prior to commencing the approach. Some aerodromes have MSAs that apply to all sectors; however, most aerodromes have different MSAs for different sectors depending on the direction from which the aircraft is arriving and the terrain over which it must cross.

2. En route charts. These show grid moras (or minimum sector safe altitudes). When determining your current MSA or grid mora altitude from an en route chart, you should take the most restrictive MSA of all the adjacent grid moras to your present position.

What is MEA?

Minimum en route altitude (MEA) is the safe altitude within the airway, i.e., 5 nautical miles either side of the airway centerline, and a minimum altitude at which radio reception is guaranteed.

What is decision height (DH)/minimum decision altitude (MDA)?

DH is the wheel height above the runway elevation at which a goaround must be initiated by a pilot unless adequate visual reference has been established and the position and approach path of the aircraft have been assessed visually as satisfactory to safely continue the approach and landing. DH is the height above the ground; i.e., it is measured off the radio altimeter or with the local QFE pressure setting off the barometric altimeter.

MDA is the altitude measured using the local QNH pressure setting; i.e., it is the height above sea level, or MDA = airport elevation + height above the ground.

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